Enrichment of palmitoleic acid by a combination of crystallization and molecular distillation Xinyi Cheng1, Yaqi Huang1, Zhuangzhuang Yang1, Tong Wang2, and Xiaosan Wang1 1Jiangnan University 2University of Tennessee Knoxville September 11, 2020 Abstract Palmitoleic acid shows a variety of beneficial properties to human health. In this study, enrichment of palmitoleic acid from sea buckthorn pulp oil by crystallization and molecular distillation was investigated. Sea buckthorn pulp oil was first converted to its corresponding mixed fatty acids (SPOMFs) that contained 27.17% palmitoleic acid. Subsequently, the effect of various factors on crystallization (i.e., crystallization temperature, solvent, ratio of SPOMFs to solvent (w/v), crystallization time) and molecular distillation (distillation temperature) were assessed on a 5-g scale. It was found that optimal crystallization conditions were a 1:15 ratio of SPOMFs to methanol (w/v) at -20 °C for 12 h, while the optimal temperature for molecular distillation was 100 °C. These conditions were utilized to obtain a liquid oil comprising 54.18% palmitoleic acid with an overall yield of 56.31%. This method has great potential for adoption by the food and medical industries for the preparation of palmitoleic acid concentrate for nutritional studies. Introduction Palmitoleic acid, or (9Z )-hexadec-9-enoic acid, is a 16-carbon omega-7 monounsaturated fatty acid (Astudillo et al., 2018). This fatty acid has demonstrated a wide range of applications in nutrition, medicine and chemical industries (Luan et al., 2018). For example, in animal models of metabolic disease, adipose tissue has been shown to release palmitoleic acid, which suppresses hepatic steatosis and improves insulin sensitivity (Trico et al., 2019). Consequently, palmitoleic acid has been in the spotlight as a promising anti-inflammatory lipid that may help ameliorate metabolic disorders (Cao et al., 2008). In addition, palmitoleic acid is used in cosmetics to improve water retention and elasticity of the skin, delay the aging of skin, hair and nails, and improve eye health (Bal et al., 2011). At present many biopharmaceutical and nutrition companies are vigorously developing palmitoleic acid-based health products and pharmaceutical preparations; some of which have been successfully marketed. Palmitoleic acid can be found in almost any oils of animal or plant origin, but usually in very low concen- trations (Wu et al., 2012). At present, wild plants are the main sources of palmitoleic acid. The seed oil of cat's claw (Doxantha unguis-cati L.), a woody vine native to the Amazon rainforest, South America and Central America, comprises 64% palmitoleic acid in the oil (Wu et al., 2012). Macadamia nut oil contains 24%-36% palmitoleic acid (Aquino-Bola~noset al., 2016), and the pulp oil from sea buckthorn contains up to 30% palmitoleic acid (Smida et al., 2019). Of these three plants, only sea buckthorn is widely distributed and has good cultivation development potential in China. From other wild plants that contain high propor- tions of palmitoleic acid, only low extraction yields of palmitoleic acid are obtained, and the geographical distribution of these plants is narrow, making them less suitable for commercial cultivation compared to sea buckthorn. Therefore, sea buckthorn pulp oil (SPO) is considered to be the best raw material for palmitoleic acid for enrichment. Posted on Authorea 11 Sep 2020 | The copyright holder is the author/funder. All rights reserved. No reuse without permission. | https://doi.org/10.22541/au.159985362.21997243 | This a preprint and has not been peer reviewed. Data may be preliminary. 1 Fatty acids can be separated by crystallization, urea complexation, supercritical fluid extraction, molecular distillation, enzymatic transesterification and preparative liquid chromatography (Lei et al., 2016; Magallanes et al., 2019; Wang et al., 2020). To date, however, very little attention has been paid to the preparation of palmitoleic acid concentrate from natural sources. Chemical synthesis is commonly used to obtain highly pure palmitoleic acid, but this creates partial trans- palmitoleic acid (Guillocheau et al., 2020). Klaas and Meurer (2004) reported the enrichment of palmitoleic acid from natural sources, in which the palmitoleic acid concentration was increased by approximately 50% in a process of transesterification, distillation and urea crystallization. However, although this led to a product that was highly enriched in the ester of palmitoleic acid (81.9%), the overall yield of this method was very low (~4%), and carcinogenic ethyl or methyl carbamate may be formed during urea inclusion (Solaesa et al., 2016), limiting the application of the extract in the food and pharmaceutical industries. In another study, Guti´errezand Belkacemi (2008) crystallized SPO product (41.4% palmitoleic acid) at 15 °C in acetone, resulting in ~53% enrichment and a 20% yield of palmitoleic acid. However, the proportion of palmitoleic acid in the liquid fraction was increased by only 27% compared with the initial proportion in crude SPO. Various methods are available in the literature for the concentration or separation of unsaturated fatty acids, but only a few are feasible for scalable preparation (Patil and Nag, 2010). In this study, crystallization and molecular distillation were used to enrich palmitoleic acid from sea buckthorn pulp oil mixed fatty acids (SPOMFs). The operating conditions, namely the crystallization temperature, solvent, ratio of SPOMFs to solvent (w/v), crystallization time, and the distillation temperature, were optimized to achieve an accep- table concentration and yield of palmitoleic acid. Importantly, these methods are suitable for the scalable production of palmitoleic acid from an inexpensive and accessible natural source. Materials and methods Materials SPO was purchased from Qinghai Kangpu Co., Ltd. (Xining, Qinghai). Standards of 37 fatty acid methyl esters were purchased from Sigma-Aldrich Chemical Co., Ltd. (Shanghai, China). Hexane, methanol, ethanol, acetone and isopropanol were provided by Sinopharm Chemical Regent (Shanghai, China). All of the other reagents were analytically pure and were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Free fatty acid preparation by saponification of SPO SPO (20 g) was combined with 2.2 g KOH and 40 mL 95% ethanol, and the saponification mixture was stirred at 75 °C in a water bath for 2 h. After the completion of the reaction, distilled water (100 mL) and hexane (200 mL) were added, and the mixture was transferred to a separating funnel to stand for separation. The unsaponifiable matters in the hexane phase were removed, and the collected lower layer containing soaps was acidified with 3 M hydrochloric acid to pH 2 to release free fatty acids. Subsequently, the solution was extracted three times with total 300 ml hexane. The upper layer containing free fatty acids was collected, and trace water was removed using anhydrous sodium sulfate. Then the extract was concentrated to dryness at 50 °C on a rotary evaporator to obtain SPOMFs. The final product was stored in a sealed aluminum pot at -20 °C before use. Crystallization of SPOMFs The crystallization of SPOMFs was performed at a controlled temperature in a 100 mL batch reactor. In general, SPOMFs of 5 g was mixed with a certain ratio of solvents, and then the mixture was allowed to crystallize at a selected temperature. At the end of the crystallization, the crystals were removed by low- temperature filtration to obtain the liquid fraction 1 that was rich in palmitoleic acid. Samples were obtained at regular time intervals, and solvent in the sample was evaporated under reduced pressure. The fatty acid composition was analyzed by GC after being diluted to 2 mg SPOMF/mL. Several parameters were optimized, which were crystallization temperature, solvent, substrate ratio of SPOMFs to solvent (w/v) and crystallization time. Five solvents were used to prepare the free fatty acid Posted on Authorea 11 Sep 2020 | The copyright holder is the author/funder. All rights reserved. No reuse without permission. | https://doi.org/10.22541/au.159985362.21997243 | This a preprint and has not been peer reviewed. Data may be preliminary. 2 solutions, and they were hexane, methanol, ethanol, acetone and isopropanol. Crystallization temperature was in the range of 0 °C to -40 °C. Six different SPOMFs-to-solvent ratios in the range of 1:3 to 1:18 (w/v) were used, and it was found that the best range for investigation was 1:9 to 1:18. The crystallization time was increased from 4 h to 16 h. While one of these parameters was being optimized, the others were held at a fixed level. After one parameter optimization was completed, the optimal value of this parameter was used for the optimization of other parameters. The design for these optimization experiments is outlined in Table 1. The yield and enrichment of palmitoleic acid content were used as the main response variables for the optimization. As a small amount of saturated fatty acids still remained in the liquid fraction 1, a secondary crystallization was conducted for further concentration of palmitoleic acid. Secondary crystallization was conducted at -40 °C by mixing 5 g liquid fraction 1 with 20 mL methanol for a period of time. The resulting crystals were separated from the liquid, giving liquid fraction 2. The fatty acid composition was analyzed (see below) and the yield of palmitoleic acid was calculated. Molecular distillation of liquid fraction 2 The palmitoleic acid in liquid fraction 2 obtained from the two-step crystallization was further enriched using molecular distillation in a standard glass evaporator (KDL 1, UIC GmbH, Alzenau-Hoerstein, Germany). Molecular distillation was performed varying the evaporation temperature from 95 °C to 110 °C to obtain a liquid containing a high proportion of palmitoleic acid. The other operation parameters were fixed, and were as follows: a rotation speed of 400 rpm, a feed temperature of 70 °C, a feed rate of 1 mL/min, a vacuum level of 10-3 mbar and a condenser temperature of 55 degC.